Wellbore operations at times require displacement of mud used in drilling with completion fluids. Additionally, the wellbore needs to be cleaned up to remove debris such as mud residue or cuttings that may remain in the wellbore. Tools such as casing scrapers and downhole filters are used for such operations. In addition, circulation at high rates can be used to not only displace the drilling mud but to also get some of the debris out of the well before trying to filter the remaining debris by, for example using tools with cup seals against the casing wall so that the well fluids are directed into a filtering tool with a screen and a debris retention volume which fills as the cleanup tool is moved out of the hole with the bottom hole assembly.
What happens at high circulation rates during fluid displacement operations is that the density varies to such an extent that wide pressure fluctuations can occur in the wellbore and in the tubing string. To the extent there are valves or other equipment that are sensitive to pressure application and removal cycles for their operation, the result can be a premature operation of downhole equipment. Another issue, when it is desired to use very high circulation rates, is that the resistance to flow increases and the operating pressure increases to get the desired flow rates. In some situations there is an exposed open hole or a formation where extensive fluid losses can occur. This cost the operator money in several ways. One is the cost of the fluid lost and another is the subsequent productivity loss from the formation that comes about from fluid loss. The higher circulation rates that bring about an increase in pressure to accomplish them also have a tendency to propel solids in the circulated fluid into small passages that are used to operate components in the bottom hole assembly and thus heighten the possibility that such equipment will not work when it is later needed for other operations. One compromise in the past has been to keep circulation rates low with the hope that the circulation pressure will be minimized but this has the undesirable effect of undermining the effectiveness of circulation as a well cleanup operation while still leaving the risks of fluid loss and equipment malfunction.
The present invention deals with these issues while providing an effective way to clean up the well. The zone that may take on fluids due to high circulation rates during wellbore cleanup is isolated with a packer. Lower portions of the completion whose operating systems may clog with debris due to high circulation pressures of debris laden mud are isolated in the tubing string during circulation. After circulation at the desired rates is concluded, the packer is unset and the string is pulled to allow a well filtering tool to capture the remaining debris in the annulus. The string is then repositioned and the tubing valve is opened to allow further downhole operations to take place. These and other aspects of the present invention will be more readily apparent to those skilled in the art from a review of the description of the preferred embodiment and the associated drawing with the understanding that the claims define the full scope of the invention.